The FBI reportedly launched a probe Wednesday to get to the bottom of Chicago prosecutors’ secret deal to drop all charges against Jussie Smollett even though they insisted they had the evidence for a conviction.

The report came as State’s Attorney Kim Foxx defended the bombshell decision to walk away from the prosecution — even as she maintained her office had an airtight case.

“Based on the facts and the evidence . . . this office believes that they could prove him guilty,” Foxx told WLS. But she added of prosecutors’ decision to let Smollett walk free with no more than the forfeiture of his $10,000 bail and 16 hours of community service, “I believe this is a just outcome based on the circumstances.”

The revelation regarding Foxx’s decision last month adds to the growing mystery behind prosecutors’ surprise decision to drop the case.

State’s attorney spokeswoman Keira Ellis said Foxx “informally separated herself from the decision-making over the case” and did so “in an abundance of caution.”

Foxx indicated she was stepping aside because, just days after the Jan. 29 alleged attack, she was put in touch with a member of Smollett’s family through Tina Tchen, a Chicago lawyer and former First Lady Michelle Obama’s chief of staff, and tried to get the FBI to take over the probe.

Foxx is under fire by critics who say she should have recused her entire office from the case and gotten a special prosecutor to take over.

But Ellis said Foxx wasn’t required to do that by law.

“The State’s Attorney did not formally recuse herself or the Office based on any actual conflict of interest,” she said. “As a result, she did not have to seek the appointment of a special prosecutor.”

Ellis also clarified, “Although we used the term ‘recuse’ as it relates to State’s Attorney Foxx’s involvement in this matter, it was a colloquial use of the term rather than in its legal sense.”

Asked whether Foxx is under investigation, the Illinois Attorney General’s Office, which investigates public corruption, declined to comment.

The FBI worked closely with Chicago police from the outset of the alleged hoax hate-crime investigation — leaving open the possibility that he could still face federal charges, such as obstruction of justice, conspiracy, mail fraud or mailing threatening communications.

The decision lies in the hands of John Lausch, who was nominated by President Trump as the US attorney of the Northern District of Illinois.